There are an estimated 100 million mines and countless millions of acres of land contaminated with unexploded ordnance (UXO) worldwide. Thus, there is a need for sensor systems and methods that can detect and identify large and small metal objects buried in soil. Such objects often are mines and UXOs.
A commonly used sensor for mine and UXO detection is the electromagnetic induction (EMI) metal detector. Conventional EMI metal detectors use either frequency-domain (FD) or time-domain (TD or pulse) eddy current methods and can detect small metal targets (such as plastic-cased low-metal content mines) at shallow depths and large metal targets (such as metal-cased high-metal content mines and UXOs) at both shallow and deep depths under a wide range of environmental and soil conditions. However, metal non-mine (i.e., clutter) objects commonly found in the environment pose a major problem in identifying mines and UXO. That is because these clutter objects create false alarms when detected by a metal detector. For time-efficient and cost-effective land clearing, the detected metal targets must be classified as to their threat potential: mine, UXO or clutter. Preferably, these metal targets need to be classified in real-time or near real-time.
There are two major issues with EMI metal detectors. The first issue is the detection of metal and the second issue is metal discrimination. In the case of detection, both time-domain (TD) and frequency-domain (FD) metal detectors are capable of detecting metal objects in the ground. However, when water, and in particular salt water, environments are encountered, TD metal detectors are preferred over FD metal detectors due to the large EMI response of the water that can mask the typically small metal signature. Therefore, FD metal detectors are not used in underwater or wet environments.
Both TD and FD metal detectors are used for metal discrimination. However, each metal detector type has operating limits when metal discrimination measurements are needed. Typically, TD metal detectors can operate over a high bandwidth compared to FD metal detectors, thus, high bandwidth targets are best resolved with TD metal detectors. In the mid bandwidth range, FD metal detectors can operate with higher power efficiency than TD metal detectors. Metal targets with a mid bandwidth response are best measured with FD metal detectors. In the low bandwidth range, TD metal detectors can operate more efficiently compared to FD metal detectors.
Since there are advantages to both types of EMI metal detectors, it would be advantageous to be able to combine the time-domain and frequency-domain methods into one metal detector in an efficient manner to take advantage of the best features of both technologies.